Isomerization of the constituents of ion/neutral complexes during the fragmentation of protonated dialkyl-substituted 1,3-diphenylpropanes

• Published in: International journal of mass spectrometry Vol. 306; no. 2; pp. 167 - 174
• Main Authors: Kuck, Dietmar, Matthias, Carsten, Barth, Dieter, Letzel, Matthias C.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.09.2011
• Subjects: Alkyl cation; Hydride ion transfer; Ion/neutral complexes; Isomerization; Metastable ion; Proton transfer; Ion/neutral complexes; Alkyl cation; Hydride ion transfer; Metastable ion; Isomerization; Proton transfer
• ISSN: 1387-3806; 1873-2798
• DOI: 10.1016/j.ijms.2010.10.007

[Display omitted] ▶ Isomerization of the neutral constituent of ion/neutral complexes ▶ Isomerization of the ionic constituent of ion/neutral complexes ▶ H +-induced 1,2- t-C 4H 9 shift affects the intra-complex reactivity of the neutral ▶ Cyclohexyl vs. methylcyclopentyl cations as constituents of I/N complexes ▶ Designed synthesis enables the generation of “tailored” I/N complexes The fragmentation of gaseous ion/neutral complexes [R +⋯C 6H 5CH 2CH 2CH 2C 6H 4–R′] with (i) R = R′ = C 4H 9, (ii) R = C 4H 9 and R′ = CH 3 and (iii) R = C 6H 11 and R′ = H has been studied by CI(CH 4)-MIKE spectrometry of the corresponding alkyl-substituted 1,3-diphenylpropanes. Different from all other isomers containing two para-alkyl substituents, the [M+H] + ion generated from the symmetrical ion [(4- tert-C 4H 9-C 6H 4)CH 2CH 2CH 2(C 6H 4-4- tert-C 4H 9) + H] + shows the characteristic fragmentation pattern of ion–neutral complexes containing a meta-alkyl-substituted 1,3-diphenylpropane. This indicates a proton-induced 1,2-shift of one or even both of the tert-C 4H 9 groups and requires the presence of the meta-( tert-C 4H 9)-substituted diphenylpropane as the neutral constituent of the eventually fragmenting I/N complex. As a consequence, it appears that the reactive complex [C 4H 9 +⋯C 6H 5CH 2CH 2CH 2(C 6H 4-3- tert-C 4H 9)] is formed prior to the generation of the expected “ para-isomer”, [C 4H 9 +⋯C 6H 5CH 2CH 2CH 2(C 6H 4-4- tert-C 4H 9)]. Isobutyl analogues, such as [(4- iso-C 4H 9-C 6H 4)CH 2CH 2CH 2(C 6H 4-4- iso-C 4H 9) + H] +, do not show evidence for the intermediacy of “isomerized” I/N complexes containing a tert-C 4H 9 + ion. The fragmentation of ion–neutral complexes containing C 6H 11 + ions, formed from the [M+H] + ions of (4-cyclohexyl)- and of 4-(1-methylcyclopentyl)-substituted 1,3-diphenylpropane, indicate that the C 6H 11 + ions only partially retain their structural identity: while the secondary isomer, (CH 2) 5 > CH +, predominantly transfers a proton in competition to hydride abstraction, indicating its stronger Bronsted acidity, the tertiary isomer, (CH 2) 4 > C +CH 3, mainly reacts by hydride abstraction. In spite of the partial isomerization, deuterium labelling experiments corroborate the usual regioselectivity of the hydride abstraction from the benzylic methylene groups in both cases.